Trichloroethylene: Difference between revisions

The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell.

Its IUPAC name is trichloroethene. In industry, it is informally referred to by the abbreviations TCE, Trike and tri, and it is sold under a variety of trade names. In medicine, it was commonly referred to as trilene and trimar during its use as a general anesthetic.

Production

Prior to the early 1970s, most trichloroethylene was produced in a two-step process from acetylene. First, acetylene was treated with chlorine using a ferric chloride catalyst at 90 °C to produce 1,1,2,2-tetrachloroethane according to the chemical equation

HC≡CH + 2 Cl₂ → Cl₂CHCHCl₂

The 1,1,2,2-tetrachloroethane is then dehydrochlorinated to give trichloroethylene. This can either be accomplished with an aqueous solution of calcium hydroxide

2 Cl₂CHCHCl₂ + Ca(OH)₂ → 2 ClCH=CCl₂ + CaCl₂ + 2 H₂O

or in the vapor phase by heating it to 300-500°C on a barium chloride or calcium chloride catalyst

Cl₂CHCHCl₂ → ClCH=CCl₂ + HCl

Today, however, most trichloroethylene is produced from ethylene. First, ethylene is chlorinated over a ferric chloride catalyst to produce 1,2-dichloroethane.

CH₂=CH₂ + Cl₂ → ClCH₂CH₂Cl

When heated to around 400 °C with additional chlorine, 1,2-dichloroethane is converted to trichloroethylene

ClCH₂CH₂Cl + 2 Cl₂ → ClCH=CCl₂ + 3 HCl

This reaction can be catalyzed by a variety of substances. The most commonly used catalyst is a mixture of potassium chloride and aluminum chloride. However, various forms of porous carbon can also be used. This reaction produces tetrachloroethylene as a byproduct, and depending on the amount of chlorine fed to the reaction, tetrachloroethylene can even be the major product. Typically, trichloroethylene and tetrachloroethylene are collected together and then separated by distillation.

Uses

Trichloroethylene is an effective solvent for a variety of organic materials. When it was first widely produced in the 1920s, its major use was to extract vegetable oils from plant materials such as soy, coconut, and palm. Other uses in the food industry included coffee decaffeination and the preparation of flavoring extracts from hops and spices. It was also used as a dry cleaning solvent, although tetrachloroethylene (also known as perchloroethylene) surpassed it in this role in the 1950s.

Due to concerns about its toxicity, the use of trichloroethylene in the food and pharmaceutical industries has been banned in much of the world since the 1970s.

For most of its history, trichloroethylene has been widely used as a degreaser for metal parts. In the late 1950s, the demand for trichloroethylene as a degreaser began to decline in favor of the less toxic 1,1,1-trichloroethane. Another problem with trichloroethylene is that it's just too good a solvent in many mechanical applications, as it easily will strip many paints almost instantly and dissolves some plastics. However, 1,1,1-trichloroethane production has been phased out in most of the world under the terms of the Montreal Protocol, and as a result trichloroethylene has experienced a resurgence in use. It has also been used for drying out the last bit of water for production of 100% ethanol.

Supplanting chloroform and ether for a significant period of time, trichloroethylene demonstrated superior efficacy in induction times and cost-effectiveness. Pioneered by ICI in Britain, its development was hailed as a revolution: lacking the great hepatotoxic liability of chloroform and the unpleasant pungency and flammability of ether, it nonetheless had several pitfalls, including the sensitization of the myocardium to epinephrine, potentially acting in an arrhythmogenic manner. Its low volatility demanded the employment of carefully regulated heat in its vaporization. Research demonstrating its transient elevation of LFTs (Liver Function Tests) raised concerns regarding its hepatoxic potential; several deaths occurred as a result, though the incidence was comparable to that of halothane hepatitis. Incompatibility with soda lime (the CO₂ adsorbent utilized in closed-circuit, low-flow rebreathing systems) presented dangers: it was readily decomposed into 1,2-dichloroacetylene, a neurotoxic compound potentially responsible for its hepatoxic potential, though its metabolite trichloroacetic acid is more probably the etiological source of the latter. Halothane usurped a great portion of its market in 1956, with its total abandonment not achieved until the 1980s, when its use as an analgesic in obstetrics was implicated in foetal death. Concerns of its carcinogenic potential were raised simultaneously.

The active metabolite of trichloroethylene is trichloroethanol, identical to that of chloral hydrate. Therefore, concerns of the carcinogenicity of the latter have been raised, and is subject to on-going debate.

Chemical instability

Although it has proven useful as a metal degreaser, trichloroethylene itself is unstable in the presence of metal over prolonged exposure. As early as 1961, this phenomenon was clearly recognized by the manufacturing industry, since an additive was instilled in the commercial formulation of trichloroethylene. The reactive instability is accentuated by higher temperatures, so that the search for stabilizing additives is conducted by heating trichloroethylene to its boiling point in a reflux condenser and observing decomposition. The first widely used stabilizing additive was dioxane; however, its use was patented by Dow Chemical Company and could not be used by other manufacturers. Considerable research took place in the 1960s to develope alternative stabilizers for trichlorethylene. The principal family of chemicals that showed promise was the ketone family, such as methyl ethyl ketone. Considerable research was conducted at Frontier Chemical Company, Wichita, Kansas on this class of ketones using reflux condensation experiments.

Health effects

Organochlorine compounds such as trichloroethylene present a potentially serious environmental liability given their great resistance to natural degradation and their high marine toxicity.

When inhaled, trichloroethylene depresses the central nervous system. Its symptoms are similar to those of alcohol intoxication, beginning with headache, dizziness, and confusion and progressing with increasing exposure to unconsciousness and death. Caution should be exercised anywhere a high concentration of trichloroethylene vapors may be present, because it quickly desensitizes the nose to its scent, and it is possible to unknowingly inhale harmful or even lethal amounts of the vapor -- that is, it has poor warning properties.

The long-term effects of trichloroethylene on human beings are unknown. In animal studies, chronic trichloroethylene exposure has produced liver cancer in mice, but not in rats. Studies on its effects on reproduction in animals have been similarly inconsistent, and so no conclusive statements about its ability to cause birth defects in humans can be made. Recent studies have shown a correlation between male fertility and exposure to trichloroethylene. Trichloroethylene has been shown to reduce sperm counts in some cases.

More recent analyses indicate low-level evidence of a mutagenic or teratogenic effect; thus, it is known that it promotes the formation of tumors, though the exact pathway is not well-understood. Its long-term safe use as a surgical anesthetic did not lead to an increased incidence of cancer as compared to background levels, indicating that any such effect is most probably extremely low-level. It is current categorized as IARC 2A, analogous to trichloromethane—reasonably anticipated to be a human carcinogen. More information on the carcinogenic potential of organochlorine compounds may be gleaned from the report on carcinogens.

The Environmental Protection Agency mounted a major effort in the 1990s to assess how dangerous trichloroethylene was to human health. Following four years of study, senior EPA scientists concluded in 2001 that it is 2 to 40 times more likely to cause cancer than the EPA had previously believed.

The National Academy of Sciences reported Thursday, July 27, 2006, that significant "evidence on [the] carcinogenic risk and other health hazards from exposure to trichloroethylene has strengthened since 2001." The report goes on to say there is "a large body of epidemiologic data available" on TCE showing the chemical is a possible cause of kidney cancer, reproductive and developmental damage, impaired neurological function and autoimmune disease. TCE is found in about 60 percent of the nation's worst contaminated sites in the Superfund cleanup program, specifically sites maintained by the United States Department of Defense, United States Department of Energy and NASA. [1]

Existing regulation

Until recent years, the US Agency for Toxic Substances and Disease Registry (ATSDR) contended that trichloroethylene had little-to-no carcinogenic potential, and was probably a co-carcinogen—that is, it acted in concert with other substances to promote the formation of tumors.

Half a dozen state, federal, and international agencies now classify trichloroethylene as a probable carcinogen. California EPA regulators consider it a known carcinogen and issued their a risk assessment in 1999 that concluded that it was far more toxic than previous scientific studies had shown.

Proposed U.S. federal regulation

In 2001, a draft report of the Environmental Protection Agency (EPA) laid the groundwork for tough new standards to limit public exposure to trichloroethylene. The assessment set off a fight between the EPA and the Department of Defense (DoD), the Department of Energy, and NASA, who appealed directly to the White House. They argued that the EPA had produced junk science, its assumptions were badly flawed, and that evidence exonerating the chemical was ignored.

The DoD has about 1,400 military properties nationwide that are polluted with trichloroethylene. The chemical has contaminated 23 sites in the Energy Department's nuclear weapons complex — including Lawrence Livermore National Laboratory in the San Francisco Bay area, and NASA centers, including the Jet Propulsion Laboratory in La Cañada Flintridge.

High-level political appointees in the EPA — notably research director Paul Gilman — sided with the Pentagon and agreed to pull back the risk assessment. In 2004, the National Academy of Sciences was given a a $680,000 contract to study the matter, releasing its report in the summer of 2006. The report has raised greater concern about the adverse health effects of TCE, opening up the debate for better regulation.

Reduced production and remediation

In recent times, there has been a substantial reduction in the production output of trichloroethylene; alternatives for use in metal degreasing abound, chlorinated aliphatic hydrocarbons being phased out in a large majority of industries due to the potential for irreversible health effects and the legal liability that ensues as a result.

The U.S. military has virtually eliminated its use of the chemical, purchasing only 11 gallons last year. About 100 tons of it is used annually in the U.S. as of 2006.

Recent research has focused on aerobic degradation pathways in order to reduce environmental pollution through the use of genetically modified bacteria. Limited success has been attained thus far; the intended application is for treatment and detoxification of industrial wastewater.

External links

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